Filtration is a process in which membranes are used to separate components in a liquid solution or suspension based on their size differences. Two types of filtration include tangential-flow filtration (TFF), also known as cross-flow filtration (CFF), and direct-flow filtration (DFF), also known as normal-flow filtration (NFF).
Tangential-flow or cross-flow filtration applications often use cassettes or other plate and frame formats. These plate and frame formats typically incorporate a plurality of flat sheet membranes arranged between external flat plates and manifolds. In tangential-flow or cross-flow filtration, the fluid to be filtered is passed through the inlet of the manifold, into the cassette, and tangentially to the first (or upstream) surface of the membranes. A portion of the fluid passes through each of the membranes from the first surface to the second (or downstream) surface, through the cassette and out one outlet of the manifold. Another portion of the fluid passes tangentially to the first surface of the membrane, through the cassette and out another outlet of the manifold without passing through the membranes. The fluid passing into the inlet of the manifold and into the cassette is commonly referred to as the feed. The feed contains various sized molecules and possibly debris. The fluid passing from the first surface of the membrane to the second surface of the membrane is commonly referred to as the filtrate. The filtrate contains the smaller molecules that have passed through the pores of the membrane. The fluid passing parallel to the first surface of the membrane without passing through to the second surface of the membrane is commonly referred to as retentate. The retentate contains the larger molecules that have not passed through the pores of the membrane.
Direct-flow or normal-flow filtration differs from cross-flow filtration in that the feed flow is directed towards the membrane, not tangentially across it. Particles that are too large to pass through the pores of the membrane accumulate at the membrane surface, while smaller molecules pass through to the filtrate side.
Conventional cassette encapsulation is achieved by interleaving multiple layers of screen mesh and membrane in a stack to be bound together as a cohesive stack, typically with a single or two-part liquid urethane or silicone. The stack may be bound by retaining the layers between plates while impregnating and encapsulating the edges of the stacked layers with the urethane or silicone. The encapsulated stack is often termed a cassette. However there are numerous problems associated with current urethane- or silicone-encapsulated cassettes.
For example, urethane- or silicone-encapsulated cassettes are associated with excessive extractables and leachables. Undesired contaminants from single and two-part liquid urethanes and silicones have a tendency to leach and extract into the process fluid during use or storage. These contaminants may or may not be hazardous when found in the process fluid or final product. In either case, however, they are generally undesired and preferably not present. Thus, there is a need in the art to minimize or eliminate these materials.
In addition, single and two-part urethane or silicone used in conventional cassettes are inconvenient and require long cure cycles, and the assembly process is labor intensive and not conducive to automated assembly. As a result, the build cycle for a traditional cassette is typically two to three days. Thus there is a need in the art for cassettes that have a shorter build cycle, preferably one day, and are adaptable to automated assembly.
The quality of conventional cassettes, as determined in part by variations in height, width and length, is also oftentimes inconsistent. As mentioned above, traditional cassette technology involves interleaving screen mesh and membrane prior to encapsulation. Each one of these materials, encapsulant, screen mesh and membrane, can contribute to variation in the final product. The variation is typically measured in terms of fluid flow performance. Slight variations in channel height or width from cassette to cassette will yield variable membrane flux (the rate of fluid flow through the membrane); assuming the membrane performance is consistent. Thus, there is a need in the art to minimize variations in flow channel dimensions (height, width and length) from layer to layer and cassette to cassette.
The use of single and two-part urethane or silicone also limits cassette height. The taller the cassette, the more likely the downward pressure created by the weight of the material will cause the encapsulant to settle at the lower portions of the cassette. As a result, the encapsulant may encroach on the ports at the lower portions of the cassette. For this reason, the viscosity of the encapsulant is critical. If the viscosity of the encapsulant is too low, the encapsulant will flow through the cassette too quickly. Conversely, if the viscosity of the encapsulant is too high, it will not flow adequately. To strike a balance, typical production-size cassettes are limited to approximate height between 0.5-inch and 4-inch (about 1 cm-10 cm), where the difference in sizes correlates to the number of membrane layers and corresponding feed and filtrate layers. Cassettes at the upper height range (4-inch or 10 cm) have approximately 5-times (5×) the membrane area (1×) found in a cassette at the lower height range (0.5 inch or 1 cm) for a given standard footprint. Creating cassettes beyond this range is limited by the encapsulant issues previously described. Thus, there is a need in the art for cassettes that are not limited in height by the use of the aforementioned encapsulants.
This limited cassette height in turn affects the number of gaskets used in a production assembly (a stack of multiple cassettes). A typical production-sized filter holder can accommodate four or five cassettes at 5× height, or twenty to twenty-five cassettes as 1× height, stacked between plates. As each cassette unit requires a gasket, the number of gaskets can quickly become unwieldy. Thus, there is a need in the art for taller cassettes, thus reducing the number of gaskets used in production.
Further, the single and two-part liquid urethane or silicone used in the assembly are subject to deformation after curing. As described above, the encapsulant also acts as a binder to hold the materials in place. In a typical assembly process, the un-encapsulated cassette is placed between two plates, and the stack is compressed prior to introducing the encapsulant. The cassette is released from the plate clamp after the encapsulant-curing cycle completes. The cassette will then relax to its free form state. As a result of this relaxing or settling, and depending upon the fixture compression, the stack can “pillow out” or expand beyond the rigid encapsulant frame. The channel height, and therefore the cassette height, is thus determined in part by the dimensions and shape of the now-cured encapsulant.
The channel height is also defined in part by the clamping force applied during encapsulation. Clamping force has an inverse relationship to channel height. An increase in clamping force results in a decrease in channel height. In some cases it is desirable to push the screen into the membrane, but not always. Therefore, there is a need in the art for a cassette-assembly process that permits the channel height to be uniformly adjusted.
Traditional cassettes require the end-user to re-compress the cassette in a filter holder or clamp prior to use. The filter holder or clamp may in turn reduce the flow channel height. Since fluid velocity at the membrane surface is critical to membrane flux, any variance in flow channel height will result in varying membrane flux. While most suppliers provide a clamping force range with their product, none of the suppliers currently offer a dead stop, i.e., a cassette that is clamped and compressed, but cannot be over-compressed. Thus, there is a need in the art for a cassette that cannot be over-compressed.
Traditional cassettes rely on gravity, pressure or vacuum, and encapsulant flow, to impregnate the edges of the stack and define the flow channel perimeter. This creates a fixed and sometimes undesirable relationship between the dimensions of the outer perimeter and the dimensions of the inner perimeter. Thus there is a need in the art for a cassette that decouples this relationship and allows the inner and outer perimeters to have significantly different shapes.
Traditional cassettes are also difficult to clean. Because conventional urethane and silicone cassettes rely on gravity, pressure or vacuum to impregnate edges of the stack with liquid encapsulant, the encapsulant will seek the path of least resistance, which causes non-uniformities in the inner encapsulant perimeter. In the final product, this non-uniformity in the perimeter results in non-uniformities in the feed and filtrate flow paths and results in areas where no-flow zones or “dead-spots” develop during use and cleaning. No-flow zones that are not cleaned adequately may grow bacteria because the offending materials left behind cannot be washed out. The no-flow zones may also decrease the efficiency of traditional cassettes. Thus, there is a need in the art for a cassette-assembly process that minimizes or eliminates no-flow zones.
While currently-available cassettes are often sold as reusable from two to fifty times, depending upon the process, reusable products require expensive validations. Therefore, in order to be accepted by the market, disposable products must cost less while delivering the same performance. Companies that lower the price of existing cassette technology while calling it a disposable will not fool savvy customers. Such circumstances will likely cause customer to reuse these “disposable” devices since they are identical to the higher priced reusable. Thus, there is a need in the art for a cassette product that is easy to manufacture, can be sold as a one-to five-use disposable, and fits existing hardware currently occupied by traditional cassette products.
The single and two-part liquid urethane or silicone used in conventional direct-flow filtration capsules are also subjected to compressive forces, and therefore are prone to the same issues and inefficiencies described above. Thus there is a need in the art for filtration capsules, and filtration capsule-assembly processes, that minimize or eliminate these problems.
The present invention alleviates or eliminates at least some of the disadvantages of the prior art. These and other advantages of the present invention will be apparent from the description as set forth below.